Gypsum wallboard slurry and method for making the same

ABSTRACT

A slurry for manufacturing gypsum board is disclosed. The slurry comprises calcined gypsum, water, a foaming agent, and a thickening agent. The thickening agent of the present disclosure acts to improve the cohesiveness of the slurry without adversely affecting the setting time of the slurry, the paper-to-core bond (wet and dry), or the head of the slurry by acting as a defoaming agent or coalescing agent. Examples of suitable thickening agents include cellulose ether and co-polymers containing varying degrees of polyacrylamide and acrylic acid. A gypsum board and method of forming the slurry and the gypsum board are also disclosed. The gypsum board comprises a gypsum layer formed from the slurry.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.14/944,539 having a filing date of Nov. 18, 2015, which is acontinuation of U.S. application Ser. No. 13/403,861 having a filingdate of Feb. 23, 2012, which claims the benefit of U.S. ProvisionalApplication No. 61/445,977 having a filing date of Feb. 23, 2011, all ofwhich are hereby incorporated in their entirety.

FIELD OF INVENTION

This disclosure generally relates to slurry formulations for theproduction of gypsum board. More particularly, this disclosure relatesto such slurry formulations that use improved thickening agents.

BACKGROUND

Gypsum board is a composite material made from two cover sheets orfacers (Face/Back) with a gypsum layer (i.e., a gypsum core) sandwichedbetween the sheets. Physical properties of the facers, facer/gypsum corebond, and strength of the gypsum core, all influence physical propertiesof the gypsum board.

Conventional methods of preparing gypsum wallboard are well known tothose skilled in the art. For example, conventional dry ingredients, wetingredients, and foam can be mixed together to create a fluid mixture or“slurry.” The dry ingredients can include, but are not limited to, anycombination of calcium sulfate hemihydrate (stucco), glass fiber, andaccelerator, retarder, and in some cases natural polymer (i.e., starch).The wet ingredients can be made of many components, including but notlimited to, a mixture of water, paper pulp, potash, and polymer(hereinafter, collectively referred to as a “pulp paper solution”). Thepulp paper solution provides a significant portion of the water thatforms the gypsum slurry of the core composition of the wallboard. Thefoam is pre-generated and continuously fed to the slurry andhomogeneously mixed with the slurry.

The slurry is discharged from the mixer through the mixer's outlet chuteor “boot”, which spreads the slurry on a moving, continuous top facingmaterial. A moving, continuous bottom facing material is placed on theslurry and the top facing material, so that the slurry is positioned inbetween the top and bottom facing materials to form the board. The boardcan then pass through a forming station which forms the wallboard to thedesired thickness and width. The board then travels along a belt linefor several minutes, during which time the rehydration reaction occursand the board stiffens (i.e., the stiffening phase). The fluidity of themix, together with the vibration of the table, will spread the slurry onthe top facing material across the board width before the foaming plate.In some cases, the vibration of the table can cause some of the foam airto leave the slurry before it reaches the forming plate.

A conventional gypsum core contains about 60% to 80% air by volume,which depends in part on the components used to form the gypsum core andthe amount and structure of foam formed during manufacture of the gypsumcore. The gypsum core is formed from a slurry, which is foamed (e.g. airis entrained as the slurry is made by the introduction of foam to theslurry to form air bubbles). As the gypsum core stiffens, the airbubbles are retained in the gypsum core to yield a gypsum core with aplurality of air voids. The size and distribution of the air voids inthe gypsum core affects gypsum board strength (e.g. nail pull) and thebonds between the facer material and the gypsum core. The bubbles/airvoids can vary in size, shape, and distribution within the gypsum core.The remaining gypsum core between the bubbles/air voids comprises gypsumcrystals that form a solid matrix between the bubbles. Typically, thewider the solid matrix between the bubbles/air voids, the stronger thegypsum core. The solid matrix is made from gypsum crystals and to a lessextent, starch. In creating the board, it will be appreciated that thereis a delicate balance between decreasing weight of the gypsum boardwhile maintaining strength of the gypsum board.

The fluidity of the stucco slurry coming from the mixer can stronglyaffect the quality of gypsum board. The stiffening and setting time ofthe slurry should be properly adjusted to keep the slurry fluid enoughto spread over the facing material and hard enough to cut at the knife.Stiffening is the change in mix fluidity caused by the hydrationreaction. The fluidity of the slurry can be controlled by the amount ofdispersant and water added to the slurry and to some extent bycontrolling the set time. The fluidity of the slurry can be increased byincreasing the water to stucco ratio and/or amount of dispersant addedto the slurry and vice versa, the fluidity of the slurry can bedecreased by decreasing the water to stucco ratio and amount ofdispersant used.

While the use of very fluid slurries can be advantageous in certainsituations (i.e., with high speed production lines), it can cause someaccumulation of bubbles at the top of the gypsum core below the backfacing material. Such accumulation can adversely affect thepaper-to-core bond and result in high soap usage to maintain the volumeof the slurry (the volume of the slurry is known as “head”). Also a veryfluid mix can result in air entrainment known in the industry as “corevoids”. In contrast to the bubbles that are imparted on the slurry bythe foam, core voids are large air pockets (several millimeters in size)that form in the slurry when the slurry has too high of a fluidity. Asthe slurry is deposited on and spreads over the facing material, theslurry can capture ambient air to form such core voids. Core voidsweaken the resulting board and can lead to defective board beingproduced. These drawbacks can be even more pronounced when unstable foamis used to create larger and more discrete bubbles in the core.

In order to overcome the drawbacks of a slurry with high fluidity, thecohesiveness of the slurry can be increased with the addition ofthickening agents. For example, pre-gelled starch can thicken the slurryand increase the slurry cohesiveness. Thickening is increasing mixcohesiveness by adding a thickening material. However, use of pre-gelledstarch in this manner results in defoaming which requires manufacturesto increase the amount of foaming agent used. Other thickening agentshave a slow thickening action and do not work quickly enough to be usedin this process.

As such, there remains an opportunity to provide improved slurries,methods of making such slurries, and methods of using such slurries tomanufacture improved gypsum boards. Such slurries can be improved byidentifying and using a thickener in a manner that improves thecohesiveness of the board and allows proper coalescing to form largerand more discrete air voids in the gypsum layer. As used throughout thisdisclosure, the terms “air bubble” or “bubble” is used to refer to thebubbles imparted on the slurry by the foam and “air void” or “void” arethe terms used to refer to the resulting voids that form from suchbubbles in the gypsum core of the finished board. Such use of the termsof “air void” or “void” shall not encompass the defects known as corevoids. As used in this disclosure, the terms “air bubble/bubble” and“air void/void” encompasses a bubble, a cavity, pocket, or a void.Further, it will be appreciated that the terms bubble and void can beused interchangeably when discussing the characteristics and size of thebubbles/voids. The use of such thickener should not adversely affect thesetting time of the slurry, the paper-to-core bond (wet and dry), or thehead of the slurry by acting as a defoaming agent.

DESCRIPTION OF THE DRAWINGS

The features and advantages of the present disclosure, and the manner ofattaining them, will be more apparent and better understood by referenceto the following descriptions taken in conjunction with the accompanyingfigures, wherein:

FIG. 1 is a scanning electron microscope (SEM) photograph of across-section of the control board for Example 1 formed with a stablefoam and illustrates a gypsum layer having a plurality of voids;

FIG. 2 is a SEM photograph of a cross-section of a board of Example 1 ofthe present disclosure formed with unstable foam and cellulose ether andillustrates a gypsum layer having a plurality of voids that are largerand more discrete;

FIG. 3 is a SEM photograph of a cross-section of a second control boardfor Example 2 and illustrates a gypsum layer having a plurality ofvoids;

FIG. 4 is a SEM photograph of a cross-section of a board of Example 2 ofthis disclosure formed with cellulose ether and a coalescing agent andillustrates a gypsum layer having a plurality of voids that are largerand more discrete;

FIG. 5 is a SEM photograph of a cross-section of a first control boardfor Example 3 formed from a slurry with a stable soap and illustrates agypsum layer having a plurality of air voids;

FIG. 6 is a SEM photograph of a cross-section of a second control boardfor Example 3 formed from a slurry with an unstable soap and illustratesa gypsum layer having a plurality of air voids;

FIG. 7 is a SEM photograph of a cross-section of the Example 3 boardformed from a slurry with an unstable soap and cellulose ether andillustrates a gypsum layer having a plurality of air voids;

FIG. 8 is a SEM photograph of a cross-section of a control board ofExample 4 of the present disclosure formed with cellulose ether and acoalescing agent and illustrates a gypsum layer having a plurality ofvoids that are larger and more discrete; and

FIG. 9 is a SEM photograph of a cross-section of a board of Example 4 ofthe present disclosure formed with co-polymer polyacrylamide and acrylicacid and illustrates a gypsum layer having a plurality of voids that aresimilar in size to the board depicted in FIG. 8.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of this disclosure is thereby intended.

The disclosure of the present application provides various slurries,gypsum boards formed from the various slurries, and methods of makingthe slurry and the gypsum board. The slurry can be used to form thegypsum board, more specifically, to form a gypsum layer of the gypsumboard. The gypsum board may also be referred to as drywall,plasterboard, gypsum wallboard, wallboard, and other similar terms. Itwill be appreciated that the gypsum board is not limited to anyparticular use, i.e., the gypsum board may be used for walls, ceilings,floors, tile-bases, soffits, and other similar uses.

According to at least one embodiment of a slurry of the presentdisclosure, stucco slurry can comprise calcined gypsum, water, a foam, athickener, and any number of suitable additives known in the art. Stuccoslurry is not limited to any particular type of calcined gypsum. Thecalcined gypsum may also be referred to in the art as calcium sulfatehemihydrate (CaSO₄·½H₂O), stucco or plaster of Paris. Examples ofsuitable calcined gypsum as well as sources, methods, and reactions forobtaining the calcined gypsum, are described in: U.S. Pat. No. 8,016,961to Martin et al.; U.S. Pat. No. 6,706,128 to Sethuraman; U.S. Pat. No.6,422,734 to Sethuraman et al.; and U.S. Pat. No. 6,783,587 toSethuraman et al.; hereinafter referred to as the incorporatedreferences, the disclosures of which are incorporated herein byreference in their entirety; so long as, the incorporated disclosuredoes not conflict with the general scope of the present disclosure.

In embodiments of the calcined gypsum of the present disclosure, thecalcined gypsum is capable of reacting with water, thereby forming areaction product comprising dihydrous calcium sulfate and typically,residual water. The reaction between the calcined gypsum and water isshown generally below:CaSO₄·½H₂O+1½H₂O→CaSO₄·2H₂O+heat

In this reaction, the calcined gypsum is rehydrated to its dihydratestate (CaSO₄.2H₂O) over a fairly short period of time. The actual timerequired for the reaction generally depends upon the type of calcineremployed and the type of gypsum rock that is used to form the calcinedgypsum. The reaction time may be controlled to a certain extent by theuse of additives such as accelerators and retarders, which are describedin more detail below. During the reaction, the slurry will generallytransition from a fluid state to a hard or “set” state as the hydrationreaction product forms/sets (the “setting time”).

The calcined gypsum can be used in various amounts. Typically, theslurry is manufactured and manipulated such that a gypsum layer formedtherefrom, i.e., the reaction product, will have a conventionalthickness, such as a thickness less than 1 inch, more typically athickness of from about ¼ to about ⅝ inch. The amount of calcined gypsumthat is present in the slurry will depend on the desired thickness. Forexample, when a ½ inch thick board is desired, the slurry will typicallycontain calcined gypsum in an amount from between about 337 to about1180 lbs/msf, alternatively about 548 to about 970 lbs/msf,alternatively from about 674 to about 843 lbs/msf. It will beappreciated that 1 msf refers to 1,000 square feet. As understood in theart, the amounts of calcined gypsum given in msf can be applied to andadjusted for various thicknesses of the gypsum layer formed from theslurry. For example, for a ⅝ inch thick board, the weight of stucco isfrom about 1785 lbs/msf to about 2040 lbs/msf. General dimensions andmanufacturing methods of gypsum boards are also understood in the artand are described further below.

Typically, the water and the calcined gypsum are reacted in a weightratio of from about 0.5 to about 1.5, alternatively from about 0.75 toabout 1.25, and alternatively from about 0.80 to about 1.0. Generally,it is desirable to provide enough water for the calcined gypsum to reactwith and to make the slurry liquid enough to spread over the facingmaterial and to fill the volume required to make the board.

The foaming agent may be any foaming agent understood in the art,including, but not limited to, the foaming agents described in theincorporated references. The foaming agent typically comprises anaqueous solution of soaps/surfactants and it might contain also asolvent such as ethanol, alcohol, water, or a combination thereof.Typically, the foaming agent comprises an anionic surfactant; however,it is to be appreciated that other types of surfactants can also beused, such as cationic surfactants, nonionic surfactants, etc. Thesurfactant solution typically comprises 30% to 60% of the active foamingagent. The foaming agent can be used to generate foam by a number ofconventional foam generating methods.

It will be appreciated that any number of foaming agents can be used inthe slurry with or without a coalescing agent. In certain embodiments,the foaming agent comprises an unstable foaming agent, such as AgentNB8515, which is an alkylesulfate compound made by Stepan Company ofNorthfield, Ill. Other foaming agents from Stepan Company can also beused (i.e., Cedepal® FA-406 or Alpha Foamer®) with or without acoalescing agent, as well as foaming agents from other companies such asThatcher TF, which is commercially available from Thatcher ChemicalCompany, Salt Lake City, Utah and Hyonic® PFM, e.g. PFM 30, which iscommercially available from Geo Specialty Chemicals of Lafayette, Ind.All of these soaps can be used with or without a coalescing agent.

The foaming agent can be used in various amounts. Typically, the foamingagent is present in the slurry in an amount of from about 0.1 to about2.0 lbs/msf, alternatively from about 0.4 to about 1.25 lbs/msf, andalternatively from about 0.5 to about 0.9 lbs/msf. It is to beappreciated that the foaming agent can comprise a combination of two ormore of the aforementioned surfactants. In certain embodiments, theslurry includes one foaming agent. In other embodiments, the slurryincludes two or more different foaming agents with or without coalescingagents.

The foaming agent may be in various forms, such as liquid, flake, orpowdered form. The foaming agent is useful to generate foam that impartsa plurality of bubbles in the slurry during formation of the reactionproduct, as understood in the art. By imparting, it is generally meantthat the foam brings bubbles into the slurry and/or forms bubbles in theslurry during formation. Generally, the foaming agent itself is frothedsuch that it includes bubbles before addition to form the slurry.Frothing can occur simply by mixing through mechanical agitation thefoaming agent, water, and air. The pre-generated foam can be added alongwith the calcined gypsum and water and/or after the calcined gypsum andwater are combined.

A thickening agent of the present disclosure acts to improve thecohesiveness of the slurry and does not adversely affect the settingtime of the slurry, the paper-to-core bond (wet and dry), or the head ofthe slurry by acting as a defoaming agent or coalescing agent. Exampleof suitable thickening agents include cellulose ether and co-polymerscontaining varying degrees of polyacrylamide and acrylic acid.

As used herein, cellulose ethers are cellulose derivatives that may beobtained by reacting cellulose with an alkyl halide or ethylene oxideand its derivatives. Examples of cellulose ethers include, but are notlimited to, methyl celluloses, ethyl celluloses; such as, hydroxyl ethylcellulose and ethyl hydroxyl ethyl cellulose, propyl celluloses; suchas, hydroxyl propyl cellulose and hydroxypropyl methyl cellulose.

According to at least one embodiment of the present disclosure, thecellulose ether is a non-ionic, water soluble, unmodified ethyl hydroxylethyl cellulose. The anhydroglucose unit is the fundamental repeatingstructure of cellulose and has three reactive hydroxyl groups. Thenumber of hydroxyl groups which react is expressed as the degree ofsubstitution (“DS”). The DS value, which falls between 0.5 and 1.0 forwater-soluble cellulose ethers, designates the average number ofhydroxyl positions on the anhydroglucose unit that has been etherified.The molar degree of substitution (MS) is the average number of bondedhydroxyalkyl groups per anhydroglucose unit. Further, the ethyl hydroxylethyl cellulose may have a variety of viscosity grades, includingbetween about 3500 to about 6500 mPas (2% solution, Brookfield LVspindle #3 at 12 rpm). Alternately, it may have a viscosity grade ofbetween about 4250 to about 6000 mPas (2% solution, Brookfield LVspindle #3 at 12 rpm).

An example of such an ethyl hydroxyl ethyl cellulose is Bermocoll 351X,which is commercially available from Akzo Nobel N.V., Amsterdam, TheNetherlands. It is a non-ionic, water soluble cellulose ether at mediumviscosity grade between 4250 to 6000 mPas (2% solution, Brookfield LVspindle #3 at 12 rpm). It is a fine powder grade and its particle sizeis ninety-eight percent less than 300 microns. Further, it has a DSvalue of about 0.9 and MS value of about 1.9, where the hydroxyl groupshave been etherified by substituting with ethoxy (using ethylenechloride) and hydroxylethoxy (using ethylene oxide) moieties. Anotherexample is Tylose® MH 60000 P6 which is an unmodified methylhydroxyethyl cellulose commercially available from S. E. Tylose GMBH &Co., Wiesbaden, Germany. The reported viscosity of this cellulose etheris 28000-34000 mPas (2% solution, Brookfield RV spindle at 20 rpm).

The cellulose ether can be used in various amounts. Typically, thelevels of cellulose ether present in the slurry is in an amount fromabout 0.02 to about 1.0 lbs/msf. Alternately, the cellulose ether may bepresent in the slurry in an amount of about 0.05 to about 0.8 lbs/msf,or about 0.1 to about 0.5 lbs/msf. Additionally, in at least oneexemplary embodiment, the thickening agent may include a secondarythickening agent, such as a pre-gelled starch, clay or otherconventional thickeners known in the art. Such a secondary thickeningagent may be present at a level of between 0.0 and 5.0 lbs/msf.Alternatively, the secondary thickening agent may be present in anamount of about 1 to about 5 lbs/msf, or about 3 to about 5 lbs/msf.

Suitable co-polymers of polyacrylamide are ones in solution with up toabout 40% of acrylic acid. Where a solution is provided without acrylicacid, a homo-polymer of acrylamide is used as the thickening agent. Themolecular weight of such co-polymers can range between about 100,000 to1,000,000 Daltons. It is preferred that a solution co-polymer ofpolyacrylamide with 10% acrylic acid be used. An example of such aco-polymer is the Superfloc P-26, which is commercially available fromKemira Group, Helsinki, Finland. In such a solution, the active polymercomprise about 19% to about 20% of the solution, has a pH between 4.2and 5.5, has a bulk viscosity of 12000 cps and a dilute viscosity of 180cps.

Co-polymers of polyacrylamide can be used in various amounts. Typically,the levels of co-polymers of polyacrylamide present in the slurry is inan amount from about 0.005 to about 0.05 lbs/msf. Alternately, theco-polymer of polyacrylamide may be present in the slurry in an amountof about 0.01 to about 0.05 lbs/msf (based on the active of thethickening agent in solution). The amounts are based on the activeingredient of a solution containing the co-polymers of polyacrylamide.Additionally, in at least one exemplary embodiment, the co-polymer agentmay include a secondary thickening agent, such as a pre-gelled starch,clay or other conventional thickeners known in the art. Such a secondarythickening agent may be present at a level of between 0.0 and 5.0lbs/msf. Alternatively, the secondary thickening agent may be present inan amount of about 1 to about 5 lbs/msf, or about 3 to about 5 lbs/msf.

Preferred thickening agents of this disclosure serve to promotethickening of the slurry immediately from introduction of the thickeningagent (the “thickening effect”) without having a strong defoamingeffect. For example, it is thought ethyl hydroxyl ethyl cellulose doesnot have a defoaming effect because it has a minimal effect on surfacetension. Whereas the surface tension of water is 72 Dynes/cm, that of a2% solution of an ethyl hydroxyl ethyl cellulose, like Bermocoll E351X,is around the mid-60's Dynes/cm, similar to the surface tension of thefoam slurry.

The slurry can also include a number of other additives understood inthe art. Examples of suitable additives include, but are not limited to,those described in the incorporated references, as well as starches,accelerators, fibers (such as paper and/or glass fibers), polymers,potash, clay, boric acid, plasticizers, fire retarders, mildewretarders, thickeners, dispersants, or a combination thereof. Theadditive component can be used in various amounts and can include one ormore of the aforementioned additives. Specific amounts of certainadditives can be appreciated with reference to the Examples sectionbelow. It is to be appreciated that the additives can be used in amountsgreater or less than those amounts specifically illustrated therein.

Further, as is taught by U.S. Pat. No. 8,016,961 to Martin et al.(“Martin”), embodiments of the slurry can include a coalescing agent tocoalesce the plurality of small air bubbles imparted by the foam tocreate larger and more discrete bubbles. In addition to the coalescingagents disclosed in Martin, it can be desired to utilize coalescingagents that have a delayed coalescing action. In other words, coalescingaction by the coalescing agent is delayed for a period of time while thecoalescing agent is in the slurry along with bubbles formed by the foam.The period of time may be less than the time it takes for the slurry tohave initial stiffening such that the coalescing agent can act.

Such coalescing agents can be selected based on the temperature at whichthe coalescing agent begins to fall or precipitate out of solution (the“cloud point”). Preferred coalescing agents have a cloud point (T_(CP))so that it is between the initial mix temperature (T₁) and the peak mixtemperature (T₂) of the slurry/reaction product. As such, the coalescingagent coalesces the plurality of bubbles in the slurry after thetemperature of the slurry reaches the cloud point (T_(CP)) of thecoalescing agent. This point is not necessarily exact, as the cloudpoint (T_(CP)) may vary, but a period of time does typically pass beforecoalescing of the bubbles begins which were imparted by the foamingagent. It is believed that coalescing action of the coalescing agentgenerally increases as the temperature of the slurry/reaction productsurpasses the cloud point (T_(CP)).

The reaction between the calcined gypsum and water is exothermic. Assuch, the slurry typically has a significant rise in temperature fromthe initial temperature (T₁) to the peak temperature (T₂) after mixing,i.e., once the reaction product starts forming. The change intemperature may be 15 to 25° C. or more from the initial temperature(T₁) to the peak temperature (T₂). By selecting a suitable coalescingagent having a cloud point (T_(CP)) within this range (T₁, T₂), bubbleformation and coalescing can be controlled in the slurry, and therefore,the reaction product. It is also possible that the temperature of theslurry is controlled in such a way that the coalescing agent can beactivated or deactivated based on the cloud point (T_(CP)) being passedor not.

It will be appreciated that any number of coalescing agents can be usedin the slurry. In certain embodiments, the coalescing agent comprises ablock copolymer surfactant such as ES8915 which is commerciallyavailable from BASF Corporation of Florham Park, N.J. The coalescingagent may also be referred to in the art as a nonionic surfactant.

The coalescing agent can be used in various amounts. Typically, thecoalescing agent is present in the slurry in an amount of from about0.01 to about 1.0 lbs per 1000 square feet (msf), alternatively fromabout 0.05 to about 0.5 lbs/msf, and alternatively from about 0.10 toabout 0.25 lbs/msf. The foaming agent and the coalescing agent aretypically present in the slurry in a weight ratio of from about 10:0.05to about 5:1.5, alternatively from about 7.5:1 to about 5:1,alternatively from about 7:1 to about 6:1.

The slurry can be formed by conventional methods understood in the art.Examples of such methods, and apparatuses for forming the slurry, aredescribed in the incorporated references. Typically, the slurry isformed using a mixer and a conveyor. The components of the slurry areprovided and added to the mixer. The mixer typically has one or morefeeds, such as a feed for dry components, e.g. the calcined gypsum, andone or more feeds for wet components, e.g. the water and the foam. Thecomponents are mixed in the mixer to form the foam slurry. Each of thecomponents can be added to the mixer in various combinations. Thecoalescing agent may be added through any of the water sources (i.e.,foam water, gauging water or pulp water), directly to the mixer,directly to the foam generator, or to the slurry discharge (i.e., the“boot”) depending on the production requirements. If included, theadditive component(s) can be added in a similar fashion. The slurry istypically fed to a conveyor having a facing material or cover sheetdisposed thereon. A forming plate skims the foamed slurry such that thereaction product is of a certain thickness.

As described above, once the calcined gypsum and water come intocontact, they generally begin reacting to form the reaction product. Thefoam imparts a plurality of bubbles in the slurry. The thickening agentupon inclusion begins to promote thickening of the slurry immediately orshortly after inclusion and in at least some embodiments prior to theslurry contacting the forming plate. The slurry is typically conveyedthrough the forming plate, as understood in the art.

Typically, a second facing material and/or cover sheet is applied to thegypsum layer to form the gypsum board; however, it is to be appreciatedthat the gypsum board may also include just one cover sheet and thegypsum layer. The cover sheet(s) can be folded to encapsulate edges ofthe gypsum layer. As understood in the art, the gypsum layer istypically sandwiched between the cover sheets. The cover sheets can beformed from various materials understood in the art, such as from paperor glass fiber. The cover sheets may be the same as or different thaneach other, and may be referred to as Face and Back sheets. Examples ofsuitable cover sheets, for purposes of the present disclosure, aredescribed in the incorporated references. As understood in the art,certain types of cover sheets may have additives or make-ups whichimpart desirable fire or mildew retarding properties.

Heat can be applied to the gypsum board to remove residual water fromthe gypsum layer. Methods of removing residual water are understood inthe art, such as by employing dryers or drying chambers. As understoodin the art, certain dispersants/plasticizers may be used to reduce theamount of water, hence reducing the eventual drying time/energy neededto produce the gypsum board.

The gypsum board can eventually be cut into various lengths. Typically,dimensions of the gypsum boards include a width of about 48 inches (˜120cm) to about 54 inches (˜137 cm), and a thickness of from ¼ inch (˜6 mm)to about 1 inch (˜25 mm), alternatively about ½ inch (˜13 mm) to about ⅝inch (˜16 mm), and alternatively from about ¼ inch to about and ⅜ inch(˜10 mm). The gypsum board may be made with different edges, forexample, with two different edge treatments: a tapered edge, where thelong edges of the board are tapered with a wide bevel at the front toallow for jointing materials to be finished flush with the main boardface; and a plain edge, used where the whole surface will receive a thincoating (skim coat) of finishing plaster. It is to be appreciated thatthe present disclosure is not limited to any particular dimension orconfiguration of the gypsum board.

The gypsum board can have various physical properties. Typically, thegypsum board has reduced weight relative to conventional gypsum boardsof the same general dimensions, due to the voids defined therein. It isbelieved that the gypsum boards also have strengths approaching or evensurpassing the strengths of conventional gypsum boards of the samegeneral dimensions.

The weight of the gypsum board produced from these types of slurrieswill be dependent on how thick the board is. For example, a ½ inch thickgypsum board typically has a weight of less than about 1500 lbs/msf,alternatively from about 1200 to 1400 lbs/msf, and alternatively fromabout 1200 to 1350 lbs/msf. The gypsum board shall also have sufficientstrength and paper-to-core bond strength to meet the requirements setforth in ASTM C1396 for wallboard. It will be appreciated that suchcharacteristics are measured by a variety of different measurements,including, but not limited to, nail pull strength, humidifieddeflection, compressive strength, and humidified paper core bondintegrity.

It will be appreciated that many changes can be made to the followingexamples, while still obtaining a like or similar result. Accordingly,the following examples, illustrating embodiments of the slurries andgypsum boards, are intended to illustrate and not to limit theinvention.

EXAMPLES

Examples of slurries and gypsum boards are formed using conventionalmethods understood in the art. As illustrated in the Figures and thebelow tables, the gypsum layers and boards of the present disclosureimprove the cohesiveness of the board without adversely affecting thesetting time of the slurry, the paper-to-core bond (wet and dry), or thehead of the slurry by acting as a defoaming agent.

Example 1

Table I below provides an example of a slurry formulation that can beused that includes cellulose ether and the improvements in the producedboard over a control wallboard that does not contain cellulose ether.

TABLE I Wet End Properties for Example 1 Sample Control Example 1SoapType Stepan 8515 Stepan 8515 Stucco Weight 1156 1192 (lbs/msf) Soap0.55 0.57 (lbs/msf) Dispersant 7.0 7.0 (lbs/msf) Acid Modified 20 20Starch (lbs/msf) Bermocoll 351X — 0.36 (lbs/msf) Accelerator 6.5 6.5(lbs/msf) Liquid Retarder 0.06 0.06 (lbs/msf) — — — Water/Stucco 0.760.78 Ratio ¼# min:sec 02:40 02:25 Slump (inches) 9.0 7.0 Board Weight1450 1493 (lbs/msf) NP (lbs) 77 81 20 Hr Peel (%) 0 0 Face 20 Hr Peel(%) 0 0 Back

All board samples were ½ inch thick.

As illustrated in Table I, soap usage did not change much between theControl and Example 1. However, the slump was reduced from 9.0″ to 7.0″resulting in a more cohesive slurry in the sample that includedcellulose ether (Bermocoll 351X). This demonstrates that the celluloseether did not have a strong defoaming effect. It also shows that thecellulose ether did not affect the setting time of the mix. It should benoted that the term “soap” is equivalent to a foaming agent.

In this embodiment, cellulose ether is a ethyl hydroxyl ethyl cellulose,commercially available from AkzoNobel Corporation. The cellulose etheracts as a thickener and helps control the slurry fluidity. It is dry fedto the mixer.

Accelerator is a ball mill accelerator and it is dry fed to the mixer.

Regular starch is acid modified corn starch and it is dry fed to themixer. The water comprises pulp water, gauging water and foam waterwherein the pulp water can be about 180 lbs/msf, the foam water canrange from about 175 lbs/msf to about 525 lbs/msf, and the gauging watercan range from about 220 lbs/msf to about 660 lbs/msf.

Additional benefits can be appreciated with reference to Table II andFIGS. 1 and 2. Using the similar formulation as set forth in Table I,the board weight was dropped to between about 1300 lbs/msf and 1360lbs/msf. The Example 1 board sample was produced from a slurry withcellulose ether and an unstable soap, which comprised Agent NB8515 inthis embodiment. In contrast to Table I, the control board was producedusing a stable soap, which comprised Thatcher TF in this embodiment. Asshown below, the sample with the cellulose ether had a higher nail pullstrength while having stronger humidified bond values; despite having alower board weight.

TABLE II Humidified Bond % Failure Bd wt Nail Pull 2 hr 20 hr Sample(lbs/msf) (LbF) Face Back Face Back Control 1550 75 85 55 70 55 (stablesoap, no cellulose ether) Example 1 1357 78 10 22 11  9 (unstable soap,cellulose ether)

Additional benefits can be appreciated with reference to FIGS. 1 and 2,which each respectively show a SEM photograph of a cross section of theControl board sample of Table II and a SEM photograph of a cross sectionof the Example 1 board sample of Table II. Referring to FIGS. 1 and 2,it can be seen that the addition of the cellulose ether with unstablesoap in Example 1 (shown in FIG. 2) also facilitates the formation oflarger air voids in the resulting board (˜100 to 350 microns) versus theair voids in the control board with stable soap (˜20 to 250 microns), asshown in FIG. 1. As discussed above, larger air voids are desired toincrease the strength of board while permitting the board weights to befurther reduced.

Example 2

Table III below provides another example of a slurry formulation thatcan be used that includes cellulose ether and demonstrates theimprovements in the produced board over two different control wallboardsamples that do not contain cellulose ether.

TABLE III Wet End Properties for Example 2 and Resulting PropertiesComponent (lbs/msf) Control 1 Control 2 Example 2 Calcined Gypsum (dry)1168 1048 1042 Foaming Agent 0.7 1.02 1.0 Coalescing Agent 0.0 0.0 0.15Potash 3.2 1.5 0.75 Accelerator 9.0 9.5 8.4 Fiberglass 1.0 1.0 1.0Regular Starch - acid modified 8.0 9.0 9.0 corn starch Starch -pregelled 0.0 0.0 5.0 Retarder 0.1 0.1 0.1 Cellulose Ether 0.0 0.0 0.5Boric Acid 0.0 0.0 0.5 Water/Calcined Gypsum Ratio 0.86 0.88 0.93 BoardWeight (lbs/msf) 1490 1344 1340 Nail Pull (lbs/msf) 84 65 81 20 HrHumidified Bond (Face/Back) 1%/3% 10%/95% 0%/1% Humidified Deflection(inches) — 0.07 0.08

All board samples were ½ inch thick.

The coalescing agent is an EO/PO reverse block copolymer, having a cloudpoint (T_(CP)) of from about 16.0 to about 60.0° C. according to ASTMD2024 and an ethylene oxide (EO) weight percent of from about 10 toabout 50 based on 100 parts by weight of the reverse EO/PO blockcopolymer, commercially available from BASF Corporation.

Accelerator is a ball mill accelerator and it is dry fed to the mixer.

Cellulose ether is ethyl hydroxyethyl cellulose, commercially availablefrom AkzoNobel Corporation. The cellulose ether acts as a thickener andhelps control the slurry fluidity. It is dry fed to the mix.

The acid modified corn starch is dry fed to the mixer.

The water comprises pulp water, gauging water and foam water wherein thepulp water can be about 180 lbs/msf, the foam water can range from about175 lbs/msf to about 525 lbs/msf, and the gauging water can range fromabout 220 lbs/msf to about 660 lbs/msf.

The control board was produced using a stable soap, which comprisedThatcher TF in this embodiment. The Example 2 with cellulose ether and acoalescing agent used a stable soap, which comprised Cedepal® FA-406 inthis embodiment.

Additional benefits can be appreciated with reference to FIGS. 3 and 4,which each respectively show a SEM photograph of a cross section of theControl 2 board sample and a SEM photograph of a cross section of theExample 2 board sample. For example, difference in the air void sizebetween Control 2 (˜50 to 300 microns), shown in FIG. 3, and the Example2 (˜400 to 800 microns), shown in FIG. 4, can be better appreciated.Example 2 is considered to have excellent void/bubble structure andphysical properties. As discussed above, larger and more discrete airvoids are desired to increase the strength of the board while allowingthe board weight to be decreased.

It is believed that these physical properties are imparted by the use ofthe coalescing agent with the cellulose ether; especially, when thecoalescing action is delayed such that it begins after a period of timehas passed, such as once the slurry is on the conveyor and/or at theforming plate. It is to be appreciated that the coalescing action maystart at any time after the slurry is formed and before the reactionproduct sets. It is also to be appreciated that two or more differentcoalescing agents may be employed such that two or more coalescingactions and respective periods of time can be employed.

Additional benefits of the use of the cellulose ether can be appreciatedwith reference to this Table IV. Using similar formulations as set forthin Table III for Example 2 with adjustments being made for themanufacturing plant producing the board, a board with the coalescingagent and cellulose ether was compared to a board with the coalescingagent and no cellulose ether. As shown in Table IV, the sample with thecellulose ether had higher nail pull strength and greater cohesivenessas shown by the lower slump measurement.

TABLE IV 2 Hr 2 Hr Humidified Humidified Nail Core Bond % Bond % Bd wtPull Slump Voids Failure Failure Sample (lbs/msf) (Lbf) (inches)Present? Face Back Sample I 1488 89 7.5 No 0 0 (with cellulose ether)Sample 2 1422 78 8.5 Yes 0 0 (without cellulose ether)

As noted in Table IV, the sample with the cellulose ether prevents theformation of core voids in the slurry. In contrast to the bubbles thatare imparted on the slurry by the foam, core voids are large air pockets(several millimeters in size) that form in the slurry when the slurryhas too high of a fluidity. As the slurry is deposited on and spreadsover the facing material, the slurry can capture ambient air to formsuch core voids. Core voids weaken the resulting board and can lead todefective board being produced. As such, care needs to be exercised inpreventing the formation of such core voids by ensuring the slurry has asufficient amount of cohesiveness. As shown in Table IV, the sampleswith cellular ether prevent the formation of such core voids.

Example 3

Table V provides another example of a slurry formulation that can beused that includes cellulose ether and demonstrates the improvements inthe produced board over different control wallboard samples that do notcontain cellulose ether.

TABLE V Wet End Properties for Example 3 Component Control 1 Control 2Control 3 Example 3 Soap Type Thatcher TF 8515 8515 8515 Stucco(lbs/msf) 1269 1255 1147 1184 Soap (lbs/msf) 0.64 0.503 0.55 0.57Dispersant (lbs/msf) 7.0 7.0 7.0 7.0 Acid Modified Starch 15.0 15 20 20(lbs/msf) Cellulose Ether — — — 0.36 (lbs/msf) Accelerator (lbs/msf) 6.56.5 6.5 6.5 Retarder (lbs/msf) 0.06 0.06 0.06 0.06 Water/Stucco Ratio0.78 0.76 0.76 0.78 Foam Weight 7.0 4.7 4.7 4.7 (lbs/ft³) ¼# min:sec02:20 02:40 02:40 02:25 Slump (inches) 7 9.0 9.0 7.0 Board Weight 15931564 1450 1493 (lbs/msf) NP (lbs) 88 88 77 81 2 Hr Peel (%) Face 100 230 8 2 Hr Peel (%) Back 100 6 18 20 20 Hr Peel (%) Face 18 0 0 0 20 HrPeel (%) Back 12 0 0 0

All board samples were ½ inch thick.

As illustrated in Table V, the control samples differed in the type ofsoap used. The Control 1 sample used the previously discussed stablesoap Thatcher TF and the Control 2 and Control 3 samples used thepreviously discussed unstable soap AgentNB8515. The Example 3 samplealso used the same unstable soap in combination with cellulose ether. Asshown in Table V, Control 3 and Example 3 are the closest boards inweight, in starch usage and in soap usage. While the soap usage did notchange much between Control 3 and Example 3, the slump was reduced from9.0″ to 7.0″ resulting in a more cohesive slurry in the sample thatincluded cellulose ether (Bermocoll 351X). This demonstrates that thecellulose ether did not have a strong defoaming effect. It also showsthat the cellulose ether did not affect the setting time of the mix.

In this embodiment, cellulose ether is a ethyl hydroxyl ethyl cellulose,commercially available from AkzoNobel Corporation. The cellulose etheracts as a thickener and helps control the slurry fluidity. It is dry fedto the mixer.

Accelerator is a ball mill accelerator and it is dry fed to the mixer.

Regular starch is acid modified corn starch and it is dry fed to themixer. The water comprises pulp water, gauging water and foam waterwherein the pulp water can be about 245 lbs/msf, the foam water canrange from about 60 lbs/msf to about 240 lbs/msf, and the gauging watercan range from about 290 lbs/msf to about 885 lbs/msf.

Additional benefits can be appreciated with reference to Table VI andFIGS. 5-7. As shown below, the Example 3 formulation with the celluloseether had a higher nail pull strength while having stronger humidifiedbond values than the control sample 3 that has a similar weight.

TABLE VI HB 2 Hr HB 20 Hr Wallboard BW Nail Pull % Failure % FailureSample lbs/msf (Avg.) Face Back Face Back Control 1 1571 88 100% 100%16% 12% Control 2 1565 88  2%  6%  0%  0% Control 3 1450 77  30%  18% 0%  0% Example 3 1493 81  8%  20%  0%  0%

Moreover, in comparing FIGS. 5, 6 and 7 to one another, it can be seenhow the combination of the unstable soap and cellulose ether creates alarger more discrete air bubble in the slurry/air voids in the gypsumcore. FIG. 5 shows a SEM photograph of a cross-section of the Control 1board sample that utilizes a stable foam. As shown in FIG. 5, the airvoids range in a variety of sizes (˜50 to 300 microns) with most of theair voids being on the smaller side. FIG. 6 shows a SEM photograph of across-section of the Control 2 board sample. As shown in FIG. 6, theunstable foam causes the air voids to coalesce so that they are largerand more discrete than those in the Control 1 board sample but the airvoids still range substantially in size. FIG. 7 shows a SEM photographof a cross-section of the Example 3 board sample. As shown in FIG. 7,the air voids are larger and more discrete than those in the Control 1board sample and are more homogenous and uniform in size than thosecontained in the Control 2 board sample. It is thought that it is theselarge, discrete and more uniform sized air voids that lead to higherstrength levels in lighter weight wallboard. While this comparison ismade in relation to the use of unstable foam, it should be noted, asdiscussed above, that a similar phenomenon is witnessed when acoalescing agent is used in association with the cellulose ether.

Example 4

As discussed in this disclosure, a solution co-polymer of polyacrylamidecan be used as the thickening agent in lieu of cellulose ether toachieve similar results. Table VII below provides another example of aslurry formulation that can be used that includes a solution co-polymerof polyacrylamide with 10% acrylic acid. Table VII demonstrates theimprovements in the produced board with the thickening agent of Example4 over a control wallboard sample containing no thickening agent(Control 1) and compares Example 4 to a control wallboard sample thatcontains cellulose ether (Control 2).

TABLE VII Wet End Properties for Example 4 and Resulting PropertiesComponent (lbs/msf) Control 1 Control 2 Example 4 Calcined Gypsum (dry)1070 1068 1070 Total Water 1030 1030 1030 Foaming Agent 0.81 0.83 0.85Coalescing Agent 0.064 0.067 0.068 Accelerator 9.2 9.5 9.5 Fiberglass1.0 1.0 1.0 Regular Starch - acid modified 9.0 9.0 9.0 corn starchStarch - Pregelled 4.0 4.0 4.0 Sugar (Dextrose) 1.25 1.25 1.25 CelluloseEther (Bermocoll 351X) 0.0 0.5 0 Solution Co-polymer 0.0 0.0 0.012(Superfloc P-26) Boric Acid 0.5 0.5 0.5 Board Weight (lbs/msf) 1370 13681369 Nail Pull (LbF) Not 83 82 measured Slump 10 9.5 9.0

All board samples were ½ inch thick.

In the Control 2 board sample, cellulose ether is a ethyl hydroxyl ethylcellulose (Bermocoll 351X), commercially available from AkzoNobelCorporation. The cellulose ether acts as a thickener and helps controlthe slurry fluidity. It is dry fed to the mixer.

In this embodiment, the Example 4 sample utilizes a solution co-polymerof polyacrylamide with 10% acrylic acid (Superfloc P-26), commerciallyavailable from Kemira Group. Just as the cellulose ether, the solutionof polyacrylamide with 10% acrylic acid acts as a thickener and helpscontrol the slurry fluidity. It is fed to the mixer in solution.

In each sample, the accelerator is a ball mill accelerator and it is dryfed to the mixer. The regular starch is acid modified corn starch and itis dry fed to the mixer. The water comprises pulp water, gauging waterand foam water wherein the pulp water can be about 245 lbs/msf, the foamwater can range from about 60 lbs/msf to about 240 lbs/msf, and thegauging water can range from about 290 lbs/msf to about 885 lbs/msf. Theboard samples were produced using a stable soap, which comprisedCedepal® FA-406.

As shown in Table VII, the slump was reduced from 10.0″ in the Control 1board sample to 9.0″ in the Example 4 board sample, which was a greaterreduction than what is seen between the Control 1 board sample and theControl 2 board sample with cellulose ether (10″ to 9.5″). As previouslydiscussed, such reduction in slump results in a more cohesive slurry inboth the Example 4 board sample that included the co-polymer ofpolyacrylamide with 10% sodium acrylate (Superfloc P-26) and Control 2board sample that included cellulose ether (Bermocoll 351X). Thisdemonstrates that both of these thickening agents did not have a strongdefoaming effect. It also shows that these thickening agents did notaffect the setting time of the mix.

Further similarities between cellulose ether or the solution co-polymerof polyacrylamide with 10% acrylic acid can be appreciated withreference to FIGS. 8 and 9, which each respectively show a SEMphotograph of a cross section of the Control 2 board sample of Table VIIand a SEM photograph of a cross section of the Example 4 board sample ofTable VII. Referring to FIGS. 8 and 9, it can be seen that the additionof the solution co-polymer of polyacrylamide with 10% acrylic acid inExample 4 (shown in FIG. 9) also facilitates the formation of larger airvoids in the resulting board (˜100 to 350 microns) similar to the airvoids in the Control 2 board sample with cellulose ether (˜100 to 350microns), as shown in FIG. 8. As discussed above, larger air voids aredesired to increase the strength of board while permitting the boardweights to be further reduced. Evidence of the strength of the resultingboards using one of the disclosed thickening agents can be seen in TableVII where Control 2 board sample and Example 4 board sample had nailpull strength in excess of 80 pounds of force.

While various embodiments of the compositions of the present disclosurehave been described in considerable detail herein, the embodiments aremerely offered by way of non-limiting examples of the disclosuredescribed herein. It will therefore be understood that various changesand modifications may be made, and equivalents may be substituted forelements thereof, without departing from the scope of the appendedclaims. Indeed, this disclosure is not intended to be exhaustive or tolimit the scope of the appended claims.

Further, in describing representative embodiments, the disclosure mayhave presented a method and/or process as a particular sequence ofsteps. However, to the extent that the method or process does not relyon the particular order of steps set forth herein, the method or processshould not be limited to the particular sequence of steps described.Other sequences of steps may be possible. Therefore, the particularorder of the steps disclosed herein should not be construed aslimitations of the appended claims. In addition, claims directed to amethod and/or process should not be limited to the performance of theirsteps in the order written. Such sequences may be varied and stillremain within the scope of the appended claims.

What is claimed is:
 1. A gypsum board comprising: a gypsum layer formedfrom a gypsum slurry having a composition comprising calcined gypsum;water; an aqueous foam; and a first thickening agent comprising apolyacrylamide; wherein the calcined gypsum is present in the gypsumslurry in an amount of from 1785 to 2040 lbs/msf and wherein the gypsumboard has a thickness of ⅝″.
 2. The gypsum board of claim 1, wherein thefirst thickening agent comprises a copolymer of polyacrylamide.
 3. Thegypsum board of claim 2, wherein the copolymer includes up to about 40%of acrylic acid.
 4. The gypsum board of claim 2, wherein the copolymeris present in the gypsum slurry in an amount of from −0.005 to 0.05lbs/msf.
 5. The gypsum board of claim 1, wherein the polyacrylamide is ahomopolymer.
 6. The gypsum board of claim 1, wherein the polyacrylamidehas a molecular weight of from about 100,000 Daltons to about 1,000,000Daltons.
 7. The gypsum board of claim 1, further comprising a secondthickening agent.
 8. The gypsum board of claim 7, wherein the secondthickening agent comprises a pre-gelled starch or day.
 9. The gypsumboard of claim 1, wherein the aqueous foam comprises an alkylsulfatecompound, water, and air.
 10. The gypsum board of claim 1, furthercomprising a coalescing agent.
 11. The gypsum board of claim 10, whereinthe coalescing agent comprises an EO/PO reverse block copolymer.
 12. Thegypsum board of claim 10, wherein the coalescing agent has a cloud pointbetween an initial temperature of the gypsum slurry and a peaktemperature of the gypsum slurry during hydration of the calcinedgypsum.
 13. The gypsum board of claim 12, wherein the initialtemperature is from about 18.0 to about 37.0° C. and the peaktemperature is from about 37.0 to about 70.0° C. during hydration of thecalcined gypsum.
 14. The gypsum board of claim 10, wherein thecoalescing agent is present in an amount of from about 0.02 to about 1.0lbs per 1000 square feet of the gypsum board.
 15. The gypsum board ofclaim 10, wherein the aqueous foam comprises a foaming agent and whereinthe foaming agent and the coalescing agent are present in a weight ratioof from about 10:0.05 to about 5:1.5.
 16. The gypsum board of claim 10,wherein the aqueous foam comprises a foaming agent and wherein thefoaming agent and the coalescing agent are present in a weight ratio offrom about 7.5:1 to about 5:1.
 17. A method of forming the gypsum boardof claim 1, the method comprising: combining the calcined gypsum and thewater to form a slurry, initiating thickening of the gypsum slurry uponintroduction of the first thickening agent comprising a polyacrylamideto the slurry, imparting a plurality of bubbles in the slurry by addingfoam generated from the aqueous foam wherein the aqueous foam comprisesa foaming agent; forming dihydrous calcium sulfate in a hydrationreaction of the calcined gypsum to form a set gypsum board.
 18. Thegypsum board of claim 1, wherein the aqueous foam comprises a foamingagent including a stable foaming agent and an unstable foaming agent.